photosynthesis

The sequence of reactions, performed by green plants and photosynthetic bacteria, in which light energy from the sun is converted into chemical energy and used to produce carbohydrates and ultimately all the materials of the plant. The photosynthetic reaction can be summarized as:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

There are two distinct phases in photosynthesis, the *light (or light-dependent) reactions and the *dark (or light-independent) reactions. In green plants and blue-green algae the light reactions involve the photolysis of water, producing hydrogen atoms and molecular oxygen. This oxygen, given off during photosynthesis, is the main source of atmospheric oxygen, essential for aerobic organisms. The hydrogen atoms produced are used to reduce NADP⁺ to NADPH and the energy released also forms ATP from ADP and inorganic phosphate (photophosphorylation). This ATP and NADPH is used up during the dark reactions in which carbon dioxide is fixed into carbohydrates (see Calvin cycle ). See also photosystems I and II.